This invention relates to reducing nitric oxide emissions from a gaseous fuel combustor. More particularly, it relates to interleaving a cooling gas between the fuel and the air used for such a combustor, at the point where the fuel and the air enter the combustion chamber.
It is well known that water vapor has a significant effect on nitric oxide production in flames burning in air. Thermal nitric oxide production has been found to be strongly dependent on the temperature of the flame and on the oxygen concentration, in a somewhat complex relationship. Water vapor reduces the flame temperature, and the water in the flame also reduces the oxygen concentration. The combination of these effects results in a large reduction in the rate of nitric oxide production.
Applying these principles to gas turbine combustors, previous investigators have injected steam into the combustor in order to reduce thermal nitric oxide emissions from the combustor. Typically, steam has been injected upstream of the main air swirler for the combustor, with the result that steam is partially pre-mixed with the combustion air. However, it has been found that injection of steam in this manner is less effective than expected. It has been observed that injection of steam by prior art methods is not as effective as injection of water, even after accounting for the water's latent heat of vaporization. To achieve the level of control of nitric oxide emissions predicted from the above principles, it has been found that it is necessary to inject more steam than expected. This additional steam may lower the system's thermal efficiency, increase consumption of demineralized water, and cause high dynamic pressures and shortened combustor life.
The present inventor has concluded that the primary reason why water injection is more effective than steam injection in reducing nitric oxide emissions, even after accounting for the water's latent heat of vaporization, is that the water droplets tend to evaporate in the flame front, where the temperature is highest. Hence, the cooling effect of the water's latent and sensible heat is greatest in the flame front and automatically occurs where it is most effective in reducing the thermal nitric oxide production rate. The present inventor has also concluded that for steam injection to be as effective as water injection, the steam should be injected in such a manner that the steam concentration within the flame front is maximized.
Accordingly, it is an object of the present invention to provide a method for reducing nitric oxide emissions from a gaseous fuel combustor.
It is a further object of the present invention to provide a method for using a cooling gas in a gaseous fuel combustor in order to reduce nitric oxide emissions therefrom.
It is also an object of the present invention to provide a method for introducing a cooling gas to a gaseous fuel combustor in such a manner that the concentration of cooling gas within the flame front is maximized.
It is still another object of the present invention to provide apparatus for reducing nitric oxide emissions that is readily adaptable to existing gaseous fuel combustors.